The fragmentation behaviour of ten 3,4-disubstituted 1,2,4-oxadiazole-5(4H)-thiones and seven 3,4-disubstituted 1,2,4-thiadiazole-5(4H)-ones studied here confirmed the earlier observations about the partial rearrangement of the former after ionization into the latter before further fragmentation. In the case of eight 3-(substituted phenyl)-4-(p-tolyl)-1,2,4-oxadiazole-5(4H)-thiones the fragmentations reflecting the above-mentioned molecular ion rearrangement show a clear correlation on the substituent a values. The electron-withdrawing substituents destabilize the molecular ion, so higher amounts of the rearranged ion [(RNCO)-N-1](+.) are obtained, A good correlation of log[R1NCO](+) against sigma was obtained (r = 0.96). Only a satisfactory correlation prevailed for log([(RNCO)-N-1](+.)/[(RNCS)-N-1](+.)) against sigma (r = 0.87). Copyright (C) 2001 John Wiley & Sons, Ltd.